This invention relates to improvements in a holding assembly for an electrical probe provided to observe one body relative to another, and in particular, to a probe holder having an improved support system.
The utilization of sensing probes to detect the position, speed, or vibrations of various types of structures is widespread in many industries. Such probes have been employed in rotary machinery to sense the position of an observed body, for example the rotor of either a turbine or a compressor, relative to a fixed point of reference, for example the probe. Conventionally, many probes of this type rely upon the Eddy current principle for measuring such things as shaft runout, vibrations, speed, and the like. An RF voltage generator is generally employed to provide a level of voltage that is proportional to the gap distance between the probe sensor element and the structure being observed. The gap distance between the sensor and the observed body must be accurately controlled so that changes in the voltage output of the RF voltage generator are linear in respect to the distance measured.
An example of probes of the type under discussion is disclosed in prior U.S. Pat. No.. 3,898,562. It has been found in some applications, that vibration of the probe holder assembly has generated spurious output signals due to movement of the probe tip. As the probe and its holder assembly are mounted in the probe mounting structure in a cantilevered manner, any vibration of the assembly will result in accentuated movement of the probe. Vibration of the probe holder assembly has resulted from the rotary machine operating at speeds approaching the natural frequency of the probe holder assembly. It has been determined that resonant conditions may be avoided by maintaining the undamped, fundamental mode natural frequency of the holder assembly above the second harmonic of maximum machine running speed. For example, to compensate for variations in different installations and to avoid excitation of the probe holder, the undamped, fundamental mode natural frequency of the assembly should be maintained at least 20 Hertz above the second harmonic of the maximum machine running speed.
The probe disclosed in the aforecited patent is externally mountable. Replacement probes may be accurately positioned while the rotary machine remains in operation. It is extremely desirable that this feature be maintained while eliminating the vibration problem resulting from operation of the machine at or near resonant conditions of the probe holder.